Are you using an OSP? If so, it is probably an outgassed byproduct
condensing on the crystal (FTIR can confirm this). It may be oxidation
effects either from the solder bath (lead oxides can create an
opalescent sheen which may appear as tarnish) or the metal can itself
(nickel on copper, for example can oxide slightly giving a slightly
orange-yellow hue to the part). Again, if the 'tarnish' is problematic,
I'd recommend sending the material to an outside lab (barring internal
capabilities)...
> Bill Davis, Ph.D.
> Diamond Multimedia Systems
> Senior Scientist
> Tel. 408.325.7868
> Cell. 408.888.5650
> e-mail: [log in to unmask]
>
>
-----Original Message-----
From: Jerry Cupples [mailto:[log in to unmask]]
Sent: Monday, January 25, 1999 10:15 AM
To: [log in to unmask]
Subject: Re: [TN] Tarnished marks on crystals
Paul Wareham asked:
> We are using a Fox type HC49U crystal on one of our boards and
after
>wave solder and washing, the body the crystal seems to be tarnished or
>discolored with a yellowish tint. It does not appear to be a residue
and
>the rest of the board is clean. Any idea what could caue this kind
of
>stain anyone? Paul Wareham DynaGen Systems Inc.
I have seen such "tarnish" many times. Even so-called stainless steels
will
in fact "rust" and/or discolor when exposed to detergent, water, and
heat.
When the SS is welded, this can cause carbide precipitation at the weld
seams and makes the metal more prone to rust.
Also, bear in mind that a "stain" on a shiny piece of metal may only be
a
monolayer thick. If you are NOT talking about a penetrating erosion of
metal at the seam, then you may be dealing with a purely cosmetic
effect.
It may be possible to deal with this cosmetic issue by changing
chemistry,
water temperature, drying speed, etc.
My opinion about water cleaning is that it is best done with simple hot
water; and that additions of detergents, wetting agents, descalers,
saponifiers, etc. cause problems in a certain proportion to those
solved.
This depends upon using a flux/paste chemistry which permits complete
removal without added solvency.
Changing your process to avoid this type of "corrosion" is a matter
which
should not be done without due consideration, and I would not assume
that
there is a real problem without knowing more than you have related in
this
question.
regards,
Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com
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